Chlorofluorohydrocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) have been used as refrigerants in refrigerating systems and as heating media in heat pumps. Chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) alone, or as azeotropic compositions or mixtures thereof, etc., are collectively called Freon or Freons. In recent years, it has been indicated that Freons released into the atmosphere deplete the ozone layer, thereby inflicting a serious adverse influence on the ecosystem, including humans on earth. Accordingly, the use and production of chlorofluorocarbons that pose a high risk of ozone layer depletion have been restricted under international agreements.
More specifically, dichlorodifluoromethane (CFC-12), mainly used as a refrigerant for domestic refrigerators, car air conditioners, turbo freezers, and container freezers, has been replaced by 1,1,1,2-tetrafluoroethane (HFC-134a) in compliance with the above-mentioned regulations.
However, more strict regulations have been implemented. For example, in the EU, two regulations, the “Regulation on Certain Fluorinated Greenhouse Gases,” and the “Directive Relating to Emissions of F-Gas from Air Conditioning Systems Fitted to Cars” (F-gas regulations), were announced in June, 2006. According to these regulations, car air conditioners installed in new model vehicles sold on the market after 2011, and those in all vehicles sold after 2017 must be configured to use a refrigerant having a GWP of not more than 150. Because HFC-134a currently used in vehicles has a GWP of 1,300, CO2, etc., have been proposed as potential replacements for HFC-134a. However, such a replacement entails various problems, such as required equipment modification and insufficient refrigeration capacity at high temperatures due to CO2 being a critical fluid. In addition, isobutane (i-C4H10), etc., which have been used as refrigerants for certain types of electric refrigerators, have not been used as replacements in all technical fields due to their very high combustibility.
In view of the above problem, there is a desire to develop a refrigerant with low GWP, which achieves performance equivalent to or better than HFC-134a in terms of energy efficiency, refrigerant characteristics (e.g., refrigeration capacity, boiling point, and pressure), etc., in LCCP (Life Cycle Climate Performance) evaluation; and which requires no or only slight modification of equipment.
In this regard, hydrofluoropropenes such as 1,2,3,3,3-pentafluoro-1-propene (HFO-1225ye) and 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf) are known as low GWP compounds having an unsaturated bond in the molecule. These compounds, which have a GWP lower than saturated HFC compounds, were expected to be less stable in the atmosphere than saturated HFC compounds.
The present inventors evaluated the stability of such hydrofluoropropenes, and found stability problems relating to hydrofluoropropenes in the presence of air (oxygen). More specifically, the evaluation results confirmed that progressive oxidative degradation occurred even in the temperature range to which refrigerant compositions may be exposed during actual use, thus forming acids such as CF3COOH and HF. Therefore, problems such as system corrosion, reduction of refrigeration capacity, capillary blockage, etc., were expected to occur.
In general, in mobile air conditioners or like devices to which a refrigerant is charged at the factory under construction management, there is little possibility of an air (oxygen) entrapment problem. However, stationary air conditioners or like devices are required to be charged with a refrigerant on site. The charging of the refrigerant relies on the contractor's management ability. Thus, entrapped air (oxygen) has been considered to be a primary cause of failures and problems, such as reduction of refrigeration capacity.
In conventional HFC refrigerants, replacement of such refrigerants can solve such a problem. However, in hydrofluoropropene-containing refrigerants, oxidative degradation of such refrigerants generates a large amount of acid, which may corrode metal parts of the system, etc., and thus necessitate equipment replacement. Accordingly, problems in installation, maintenance, etc., may occur with the use of hydrofluoropropene-containing refrigerants. Therefore, to use a hydrofluoropropene as a component of refrigerant compositions, a technique for enhancing the stability of the refrigerant compositions in the presence of air (oxygen) is necessary.
For example, as a stabilization technique, Patent Literature (PTL) 1 discloses that a stabilizer such as a phenolic compound, thiophosphate, benzoquinone, or aryl alkyl ether is added to a fluoroolefin.